A reduction projection type exposure apparatus known as a stepper is used in photolithographic techniques in which fine patterns of integrated circuits are exposed and transferred onto wafers such as those made of silicon. A stepper optical system contains an illumination optical system and a projection optical system. The illumination optical system illuminates light from a light source uniformly onto a reticle on which an integrated circuit pattern is drawn. The projection optical system projects and transfer the integrated circuit pattern of the reticle onto the wafer, typically by reducing the circuit patterns projection to one fifth its original size. These stepper optical systems are widely used in large scale integration (LSI) and very large scale integration (VLSI) photolithography operations.
In recent years, large scale integration (LSI) and, more recently, very large scale integration (VLSI) have rapidly become more highly integrated and functionalized. In the field of logical VLSI, larger systems are being required with the shift to system-on-chip. With this progress, finer processability and higher integration capabilities are required for a wafer, such as that made of silicon, which constitutes a substrate for VLSI. Indeed, the advance from LSI to VLSI has gradually increased the capacity of DRAM from 1 KB through 256 KB, 1 MB, and 4 MB, with the corresponding processing line width required for the stepper becoming increasingly finer from 10 .mu.m through 2 .mu.m, 1 .mu.m, 0.8 .mu.m and 0.3 .mu.m. Accordingly, it is necessary for a projection lens of the stepper to have a high resolution and a great depth of focus.
The resolution and the depth of focus of an optical lens is determined by the wavelength of the light used for exposure and the numerical aperture (N.A.) of the lens. The resolution and the depth of focus are expressed by the following equations: EQU resolution=k1.times..lambda./N.A. EQU depth of focus=k2.times..lambda./N.A..sup.2
where k1 and k2 are constants of proportionality. The resolution of the transfer pattern is proportional to the number of apertures of the projection optical lens system and is inversely proportional to the wavelength of the light from the light source. Thus, higher resolutions may be obtained by either increasing the numerical aperture or making the wavelength shorter.
From a practical standpoint, it is desirable to use a shorter wavelength of light to produce higher resolutions of transfer patterns. When the numerical aperture of the lens is increased, the angle of the refracted light is also increased preventing the capture of the refracted light. In contrast, when the exposure wavelength .lambda. becomes shorter, the angle of the refracted light becomes smaller in the same pattern, thereby allowing the numerical apperature to remain small. Furthermore, the number of apertures of the lens is limited by the lens production process, hence shortening of the wavelength is the only way to increase the resolution.
In order to achieve higher resolution of the transfer patterns, the wavelength of light sources used in a stepper is becoming shorter, going from g-line (436 nm) to I-line (365 nm) and further to KrF excimer laser beam (248 nm) and ArF excimer laser beam (193 nm). Indeed, as mentioned above, production of VLSI such as DRAM with storage capacity of more 4 MB requires the line-and-space, an index for stepper resolution, to be no more than 0.3 .mu.m. This high degree of resolution, requires the use of ultraviolet and vacuum ultraviolet wavelengths of no more than 250 nm such as those achieved with an excimer laser light source.
A stepper typically contains a combination of numerous optical members such as lenses. Unfortunately, even when each lens element in the stepper has a small transmission loss, such a loss is multiplied by the number of the lens sheets used. Accordingly, it is necessary for the optical member to have a high transmittance. However, large light transmission losses occur when using a wavelength which is shorter than the I-line. Indeed, light in the wavelength region below 250 nm ceases to transmit through most optical glass materials. Only crystalline materials or silica glasses have proven to have sufficient light transmission properties for use as optical members in excimer laser light source steppers. Among these, silica glass is widely used not only for an excimer laser stepper but also for an optical system using general ultraviolet and vacuum ultraviolet light.
Silica glasses used as optical members in photolithography apparatus are required to meet exacting specifications. Indeed, high uniformity of the refractive index distribution is required in order to reduce the amount of multiple refraction, or to reduce inner strain (birefringence) of the optical member. For example, a refractive index distribution is required to be of an order of no more than 10.sup.-6 in an apperature having a diameter of 200 nm. Furthermore, a high transmission for the silica glass is also required. Typically, lenses having large curvatures are needed for aberration correction in projection optical systems, often causing the total optical path length in the projection optical system to exceed 1000 mm. In order to maintain the throughput of such a projection optical system at 80% or more, an internal transmittance per 1 cm of the optical member needs to be 99.8% or higher, i.e., no more than 0.002 cm.sup.-1 converted in terms of inner absorption coefficient. Moreover, such a high transmittance is required to be maintained over the entire area of the optical member. For these reasons, only high purity silica glasses may be used in a optical system such as an excimer laser stepper. Thus, a need exists for silica glasses capable of being used in these optical systems.
Synthetic silica glasses are roughly classified according to production method into synthetic silica glass and fused silica glass. The production of synthetic silica glass is further classified mainly into the Vapor Phase Axial Deposition (VAD) method, which is also known as the soot re-melting method; the direct method, which is also known as the flame hydrolysis method; and the plasma method. All of these synthetic methods belong to a general manufacturing category known as a gas phase synthetic method.
In the Vapor Phase Axial Deposition (VAD) method, a high purity gaseous silicon compound is hydrolyzed in an oxygen/hydrogen flame, and deposits soot on a target. This result in a soot ingot, which is sintered at 800.degree. C. and consolidated by heating the soot ingot at a relatively low temperature of 1600.degree. C. while performing a dehydration process with chlorine gas. A silica glass ingot is then obtained.
In the direct method (flame hydrolysis method), a high purity gaseous silicon compound, such as silicon tetrachloride, is hydrolyzed in an oxygen and hydrogen flame to form minute silica glass particles (soot particles). The gaseous silicon compound, oxygen and hydrogen are expelled from a burner. A silica glass ingot is obtained by depositing to soot particles on a target, melting the soot particles to form glass particles, in a single step, while the target is being rotated, rocked and/or lowered in the direction of the burner. Using a direct method, an attempt has been made to obtain an even more uniform silica glass by performing a secondary heat treatment of about 2000.degree. C. on the silica glass optical member which is obtained by the direct method. In that method, the subsequent heat treatment is termed "secondary" as opposed to the process of synthesizing the silica glass which is the first process.
In the plasma method, a high purity, gaseous silicon compound is oxidized in a high frequency plasma flame of oxygen and argon to form the soot. A silica glass lump is obtained by depositing the soot onto a target, melting it, and making it transparent, all at once, while the target is being rotated and lowered in the direction of the burner. In general, synthetic silica glasses are obtained by using a VAD method or direct method rather than a plasma method.
Using such gas phase methods, it is possible to obtain a silica glass optical member with higher purity, higher light transmittance for wavelengths below 250 nm, larger aperture diameter, and more uniformity than obtained from fused silica glass. Fused silica glass is obtained by electric or flame melting of a natural quartz powder. For these reasons, a synthetic silica glass is viewed as a promising material for a photolithography apparatus optical system such as an excimer laser stepper.
However, the synthetic silica glasses described above are susceptible to degradation when exposed to ultraviolet light typically used in a stepper. When synthetic silica glass is exposed for a long period of time to high output ultraviolet light or excimer laser beam an absorption band of 215 nm often appears due to a structural defect known as E'-center. In addition, an absorption band of 260 nm caused by a structural defect known as NBOHC (Non-Bridging Oxygen Hole Center), may also appear. The presence of either absorption band results in a rapid transmission loss in the ultraviolet wavelength region. An E'-center represents a structure of .tbd.Si.cndot., in which .tbd. indicates bonding with three oxygen atoms rather than a triple bond and .cndot. indicates unpaired electron. An NBOHC is a structure corresponding to .tbd.Si--O.cndot.. These structural defects result in transmission losses in the ultraviolet wavelength region and render a synthetic silica glass unsuitable for use with an ultraviolet light or excimer laser.
Examples of precursors which generate structural defects include a .tbd.Si--Si.tbd., a .tbd.Si--O--O--Si.tbd., and Cl. Silica glasses produced by the plasma method or by the VAD method are known to contain such precursors. However, absorption measurements using vacuum ultraviolet, ultraviolet, visible light and infra-red spectrometer have verified that an imperfect structure due to an oxygen deficiency or an excess of oxygen generally does not occur in synthetic silica glass produced by the direct method. Moreover, using a direct method a synthetic silica glass, a high degree of purity may be obtained with the concentration of metal impurities, such as, Mg, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Co, Mn, being less than 20 ppb. Consequently, a synthetic silica glass obtained by the direct method is generally considered a promising optical member for use in excimer laser stepper. Unfortunately, synthetic silica glass produced by a direct method of the prior art often undergoes a reduction in transmittance due to the presence of unidentified precursors in the final glass.
Attempts to prevent the reduction in transmittance of a synthetic silica glass have been made. One such attempt is described in Japanese Laid-Open Patent Publication 1-201664, incorporated herein by reference, which teaches heat treating the silica glass in a hydrogen atmosphere. Another technique is described in Japanese Laid-Open Patent Publication 3-109233, incorporated herein by reference, which teaches hydrogen molecule doping. However, doping silica glass with hydrogen molecules repairs, but does not eliminate, structural defects caused by irradiation of ultraviolet light. For example, hydrogen molecules react with an E' center and are transformed to .tbd.Si--H bonds, helping to reduce the E' center concentration. However, .tbd.Si--H is rapidly transformed back to an E' center defect upon further irradiation with ultraviolet light. It is important to prevent generation of structural defects such as an E' center or NBOHC by eliminating or reducing the amount of materials which cause these structural defects.
Attempts have also been made to control the generation of structural defects such as E' center or NBOHC by reducing or excluding the presence of Cl in a synthetic silica glass. By doing this, precursors related to Cl are reduced or excluded with a certain degree of control against excimer laser or ultraviolet light irradiation defects being achieved. However, although conventional techniques, such as those described above, have shown some degree of success, often these techniques failed to reduce degradation of silica glass when using an excimer laser stepper. The invention described below aims to solve the shortcoming of conventional techniques described by providing a synthetic silica glass optical member which controls the generation of ultraviolet light defects and prevents a decline in transmittance even when the subjecting silica glass optical member to ultraviolet light and excimer laser beam of short wavelength and of high output for long durations.
Another approach to preventing the formation of structural defects has been to add fluorine to the silica glass via a VAD method. In contrast to Si--Si bonds, Si--F bonds have a large bonding energy and are not rapidly dissociated by ultraviolet light. A method of manufacturing a silica glass containing fluorine, hydroxyl group and hydrogen molecules by the VAD method is described in Japanese Laid-Open Patent Publications 8-67530 and 8-7590, incorporated herein by reference. The references disclose that by doping the silica glass with fluorine to form Si--F bonds, the number of Si--Si bonds, which are oxygen deficient type defects causing an absorption of light at 163 nm, may be reduced. Indeed, in Japanese Laid-Open Patent Publication 6-156302, incorporated herein by reference, it was discovered that some silica glasses formed by a VAD method in which fluorine concentration is no less than 100 ppm were effective as vacuum ultraviolet-use optical members. However, some silica glasses containing 100 ppm formed by the VAD method were found to generate an emission band having a peak wavelength of 585 nm and suffered a decreased initial transmittance at wavelengths below 250 nm. Thus, silica glass formed by previous VAD methods have not proven satisfactory for photolithography techniques requiring the use of ultraviolet light less than or equal to 250 nm.
The VAD method generally requires secondary processing such as heat processing in a hydrogen atmosphere. Without such heat treatment it was impossible to have fluorine, hydroxyl group and hydrogen molecules present at the same time in the silica glass. However, problems also arise during this heat treatment. The silica glass may become contaminated with impurities resulting in ultraviolet light transmission losses. Also, hydrogen diffuses into the interior of the glass substrate. This diffusion creates a non-uniform distribution of the hydrogen in the silica glass with a higher hydrogen molecules concentration occurring at the edges of the glass as compared to the center of the glass. This effect is exacerbated as the diameter of the glass increases. Thus, sufficient silica glass resistance to ultraviolet light degradation is not obtained when using previous VAD methods.
Previous attempts at fluorine doping of silica glass have been performed using the VAD method. This is because the use of the direct method with its oxygen/hydrogen flame causes fluorine contained in a dopant gas to react with hydrogen contained in oxygen/hydrogen flame to form hydrofluoric acid which is expelled out of the system. In other words, fluorine and hydroxyl groups cannot co-exist at high temperature. This can be seen from a free energy point of view because the Gibbs free energy sign for the reaction of fluorine and hydroxyl group reverses around 1200 K. Therefore, fluoride doping using a direct method performed at a high temperature of 2000 K or more will cause hydroxyl group and fluorine to react, preventing fluorine doping of the silica glass.
Even with such attempts, there remains a need to produce high quality synthetic silica glass which may be used in optical devices, such as steppers. Moreover, there remains a need for synthetic silica glasses which may be used with ultraviolet (UV) light and excimer laser beam but which resist forming defects associated with that use.